Navigation and chart work
This domain tests the paper-and-electronic mechanics of position, tide and passage planning: reading a chart correctly, converting a water track into a course to steer, and knowing what a buoy is telling you. Expect short calculations with a plotted answer.
The chart itself
- Soundings and charted depths are reduced to chart datum, normally Lowest Astronomical Tide, so the figure is the least water you can expect.
- Soundings are in metres and decimetres; drying heights sit above datum and are marked differently (often underlined).
- Depth available = charted depth + height of tide; under-keel clearance = that total minus draught minus a allowance for squat, swell and chart accuracy.
Course to steer, set and drift
A course steered through the water is a vector; the tidal stream is a second vector; the result over the ground is the third. To find the course to steer, lay off the water speed from the destination, the tidal vector from its far end, and close the triangle back to the start.
Buoyage and planning
- IALA Region A (most of the world): red to port on entering. Region B (Americas, Japan, Korea, Philippines): red to starboard on entering.
- Passage planning follows appraisal, planning, execution and monitoring; on ECDIS, set the safety contour, safety depth and safety frame deliberately rather than accepting defaults.
Worked example
You need to make good 090°T at 10 knots through the water; the tidal stream sets 180°T at 2 knots. Draw the 10-knot vector along 090°, then the tide vector south from its end. The closing vector gives a ground speed of √(10² + 2²) ≈ 10.2 knots, and the course to steer is 090°T plus arctan(2/10) ≈ 11° to starboard, so steer about 101°T. Then check a second worked case on height of tide: charted depth 2.1 m, draught 3.5 m, HW 10:00 at 4.2 m, LW 16:00 at 1.0 m, range 3.2 m. At 13:00, three hours after high water, the rule of twelfths says six-twelfths of the range has fallen, so height ≈ 4.2 − 1.6 = 2.6 m, available depth 4.7 m, under-keel clearance 1.2 m.
Traps that cost marks
- Applying the tidal stream as a course alteration instead of as a vector - the error grows with the ratio of tide to speed.
- Forgetting that drying heights are above datum and must be subtracted, not added.
- Using the charted depth as the actual depth without tide.
- Assuming Region A everywhere: examiners use American or Japanese charts deliberately.